diff --git a/src/data-sturcture.js b/src/data-sturcture.js
new file mode 100644
index 0000000..8003f12
--- /dev/null
+++ b/src/data-sturcture.js
@@ -0,0 +1,23 @@
+class DefaultMap extends Map {
+    constructor(defaultFn, entries) {
+        super(entries);
+        this.default = defaultFn;
+    }
+
+    get(key) {
+        if (!this.has(key)) this.set(key, this.default());
+
+        return super.get(key);
+    }
+
+    pop(key) {
+        const ret = this.get(key);
+        super.delete(key);
+
+        return ret;
+    }
+}
+
+module.exports = {
+    DefaultMap: DefaultMap
+};
diff --git a/src/problem1.js b/src/problem1.js
index 9a14f2c..05d8ca0 100644
--- a/src/problem1.js
+++ b/src/problem1.js
@@ -1,6 +1,24 @@
 function problem1(pobi, crong) {
-  var answer;
-  return answer;
+  
+  [p, c] = [pobi, crong].map(pages => {
+    if (pages[0] + 1 != pages[1]) return -1;
+    
+    return Math.max(
+      ...pages.map(page => {
+        var nums = page.toString().split("").map(Number);
+
+        if (nums.includes(0)) {
+          return nums.reduce((sum, cur) => sum + cur, 0);
+        }
+        
+        return nums.reduce((mul, cur) => mul * cur, 1);
+      })
+    );
+  });
+  
+  if ([p, c].includes(-1)) return -1;
+  
+  return p == c ? 0 : (p < c ? 2 : 1);
 }
 
 module.exports = problem1;
diff --git a/src/problem2.js b/src/problem2.js
index cebd07c..7e485d5 100644
--- a/src/problem2.js
+++ b/src/problem2.js
@@ -1,5 +1,11 @@
 function problem2(cryptogram) {
-  var answer;
+  let answer;
+
+  while (cryptogram != answer) {
+    answer = cryptogram;
+    cryptogram = cryptogram.replace(/(.+)\1+/g, '');
+  }
+
   return answer;
 }
 
diff --git a/src/problem3.js b/src/problem3.js
index 1baed28..9cf1c51 100644
--- a/src/problem3.js
+++ b/src/problem3.js
@@ -1,5 +1,10 @@
 function problem3(number) {
-  var answer;
+  const answer = [...Array(number).keys()]
+    .reduce((sum, cur) => {
+      return (cur + 1).toString().split("")
+        .filter(num => "369".includes(num)).length + sum;
+    }, 0);
+
   return answer;
 }
 
diff --git a/src/problem4.js b/src/problem4.js
index ee1d3bd..a6f163e 100644
--- a/src/problem4.js
+++ b/src/problem4.js
@@ -1,5 +1,11 @@
 function problem4(word) {
-  var answer;
+  const answer = word.replace(/[a-zA-Z]/g, char =>
+    String.fromCharCode(
+      char.toLowerCase() == char
+        ? 219 - char.charCodeAt()
+        : 155 - char.charCodeAt()
+    )
+  );
   return answer;
 }
 
diff --git a/src/problem5.js b/src/problem5.js
index 9368e87..a42dd5c 100644
--- a/src/problem5.js
+++ b/src/problem5.js
@@ -1,5 +1,10 @@
 function problem5(money) {
-  var answer;
+  const answer = [50000, 10000, 5000, 1000, 500, 100, 50, 10, 1]
+    .map(div => {
+      [ret, money] = [Math.floor(money / div), money % div];
+      return ret;
+    }
+  );
   return answer;
 }
 
diff --git a/src/problem6.js b/src/problem6.js
index 3f842b5..1b7ca2f 100644
--- a/src/problem6.js
+++ b/src/problem6.js
@@ -1,5 +1,30 @@
+// LCS(Longest Common Substring) Algolrithm
+const lcs = (a, b) => {
+  const dp = Array(a.length + 1).fill().map(() => Array(b.length + 1).fill(0));
+
+  for (let i = 1; i <= a.length; i++) {
+    for (let j = 1; j <= b.length; j++) {
+      dp[i][j] = a[i - 1] === b[j - 1] ? dp[i - 1][j - 1] + 1 : 0;
+      if (dp[i][j] > 1) return true;
+    }
+  }
+  
+  return false;
+};
+
 function problem6(forms) {
-  var answer;
+  const answer = [
+    ...forms.reduce((dup, [email1, nick1], i) => {
+      forms.slice(i + 1).forEach(([email2, nick2]) => {
+        if (lcs(nick1, nick2))
+          dup.add(email1).add(email2);
+      });
+
+      return dup;
+    },
+    new Set())
+  ].sort();
+
   return answer;
 }
 
diff --git a/src/problem7.js b/src/problem7.js
index ee1bb9d..4c1a569 100644
--- a/src/problem7.js
+++ b/src/problem7.js
@@ -1,6 +1,32 @@
+const { DefaultMap } = require('./data-sturcture');
+
 function problem7(user, friends, visitors) {
-  var answer;
-  return answer;
+  const graph = new DefaultMap(() => []);
+  
+  friends.forEach(([u, v]) => {
+    graph.get(u).push(v);
+    graph.get(v).push(u);
+  });
+  
+  const answer = [...graph.keys()].reduce((scores, key) => {
+    if (key === user || graph.get(user).includes(key)) {
+      return scores.set(key, undefined);
+    }
+
+    const score = graph.get(key).reduce((acc, val) => {
+      return acc + graph.get(user).includes(val);
+    }, 0) * 10;
+
+    return scores.set(key, score);
+  }, new DefaultMap(() => 0));
+  
+  visitors.forEach(key => answer.set(key, answer.get(key) + 1));
+
+  return [...answer]
+    .filter(([_, score]) => !isNaN(score))
+    .sort((a, b) => b[1] - a[1])
+    .slice(0, 5)
+    .map(a => a[0]);
 }
 
 module.exports = problem7;